Export cache, eviction, upstream and transform metrics (closes #85)

New pixa_ metrics, none with labels:
- source cache hits and misses, counted once per variant made or checked
- cache usage and limit gauges; usage is set each time eviction reads it
- eviction count and bytes, from the size_bytes the deleted row returns
- upstream fetch duration histogram and fetch error counter
- transform duration histogram and an in-progress gauge, both including
  the wait for a processing slot

Removing a damaged file during reconciliation is not counted as an
eviction. README lists every metric; the metrics config comment now says
the pixa_ metrics are measured whether or not /metrics is served.

Model: opus-5-5

README.md

@@ -1167,10 +1167,33 @@ Each item below is one setting, named by its config key:
     - the Go runtime and process metrics
     - `pixa_cache_hits_total`: image requests served from the cache
     - `pixa_cache_misses_total`: image requests not served from the cache
+    - `pixa_source_cache_hits_total`: variants made or checked whose source was
+      in the cache
+    - `pixa_source_cache_misses_total`: variants made or checked whose source
+      was not in the cache
+    - `pixa_cache_usage_bytes`: the bytes of sources and variants in the cache,
+      as eviction last read them; eviction runs after each store and every five
+      minutes
+    - `pixa_cache_max_bytes`: the cache size limit, `cache_max_bytes`
+    - `pixa_cache_evictions_total`: sources and variants removed by eviction
+    - `pixa_cache_evicted_bytes_total`: the bytes those evictions freed
     - `pixa_upstream_fetches_total`: fetches of a source image that read any
       bytes
     - `pixa_upstream_fetch_bytes_total`: the bytes those fetches read
+    - `pixa_upstream_fetch_duration_seconds`: a histogram of how long those
+      fetches took, up to the last byte of the response
+    - `pixa_upstream_fetch_errors_total`: fetches of a source image that failed,
+      before or while reading the response
     - `pixa_transforms_total`: images converted
+    - `pixa_transform_duration_seconds`: a histogram of how long those
+      conversions took, including the wait for a processing slot
+    - `pixa_transforms_in_progress`: images being converted or waiting for a
+      processing slot
+
+    pixa measures the `pixa_` metrics whether or not `/metrics` is served. They
+    have no labels, so their number never grows with the hosts and paths pixa
+    serves. Requests that share work (see Limits) count one source cache hit or
+    miss between them.
 
     A request's `handler` label is the pattern of the route it matches, such as
     `/v1/image/*`, never its path. Requests that match no route share the label